Process for materials for aligning liquid crystals and liquid crystal optical elements

ABSTRACT

A process for aligning liquid crystals adjacent to a surface of an optical alignment layer comprising: exposing at least one optical alignment layer, comprising anisotropically absorbing molecules, to polarized light; the polarized light having a wavelength within the absorption band of said anisotropically absorbing molecules; wherein the exposed anisotropically absorbing molecules induce alignment of a liquid crystal medium at an angle + and − with respect to the direction of the polarization of the incident light beam and along the surface of the optical alignment layer; and applying a liquid crystal medium to said optical alignment layer; wherein said anisotropically absorbing molecules consist essentially of diary ketones, is described. The invention also is directed to a liquid crystal optical storage medium, a liquid crystal display element, and a liquid crystal diffractive optical element made by the process of the invention and to novel polyimide compositions that are useful as optical alignment layers in the process of the invention.

This application is a division of application Ser. No. 08/886,560, filedJul. 1, 1997, now U.S. Pat. No. 5,965,691 which is a division ofapplication Ser. No. 08/624,945, filed Mar. 29, 1996, now U.S. Pat. No.5,807,498.

This invention was made with United States Government support undercooperative agreement No. 70NANB4H1525 awarded by the United StatesDepartment of Commerce. The United States Government has certain rightsin the invention.

BACKGROUND OF INVENTION

The present invention relates to processes for aligning liquid crystals,compositions useful for generating alignment of liquid crystals andliquid crystal optical elements.

Liquid crystal compounds are used in human and machine readabledisplays, fining applications in instrument controls, such as those inmotor vehicles, avionics, medical devices, process control devices andwatches. Display devices are primarily comprised of liquid crystal cellshaving a glass or other substrate coated with a transparent conductivematerial in front and behind a liquid crystal medium. Light transmissionthrough these devices is controlled through orientation of the liquidcrystal compounds or dyes dissolved therein. In this way, a voltage or,in some instances, a magnetic field may be applied to the cell so thatthe liquid crystals are oriented in a fashion such that all, some ornone of the light is passed through. In addition, depending on thedevice geometry, polarizers may be used in conjunction with the liquidcrystal medium to control light transmission.

Aligned liquid crystal cells in commercial use are typically oriented indirections suitable for controlling light transmission. That is, themolecules in the liquid crystal composition are aligned so as to assumea homogeneous or homeotropic alignment. Without external stimuli thedisplay will either appear opaque or transparent. By applying anelectric field the molecules are rotated along a fixed axis so as toalter the transmission properties in a desired fashion.

Current liquid crystal display elements include a product that utilizesa twisted nematic mode, i.e. having a structure wherein the aligningdirection of nematic liquid crystal molecules is twisted by 90° betweena pair of upper and lower electrode substrates, a product utilizing asupertwisted nematic mode, utilizing a birefringent effect, i.e. havinga structure wherein the aligning direction of nematic liquid crystalmolecules is twisted by 180° to 300°, a product utilizing aferroelectric liquid crystal substance or an antiferroelectric liquidcrystal substance. Common to each of these products is a liquid crystallayer disposed between a pair of substrate coated with a polymericalignment layer. The polymeric alignment layer controls the direction ofalignment of the liquid crystal medium in the absence of an electricfield. Usually the direction of alignment of the liquid crystal mediumis established in a mechanical buffing process wherein the polymer layeris buffed with a cloth or other fibrous material. The liquid crystalmedium contacting the buffed surface typically aligns parallel to themechanical buffing direction. Alternatively, an alignment layercomprising anisotropically absorbing molecules may be exposed topolarized light to align a liquid crystal medium as disclosed in U.S.Pat. Nos. 5,032,009 and 4,974,941, both entitled “Process of Aligningand Realigning Liquid Crystal Media.” both of which are herebyincorporated by reference.

The process for aligning liquid crystal media with polarized light is anoncontact method of alignment which can reduce dust and static chargebuildup on alignment layers. Other advantages of the optical alignmentprocess include high resolution control of alignment direction and highquality of alignment.

Requirements of optical alignment layers for liquid crystal displaysinclude low energy threshold for alignment, transparency to visiblelight (no color), good dielectric properties and voltage holding ratios,long-term thermal and optical stability and in many, but not all,applications, a controlled uniform pre-tilt angle.

The process for aligning liquid crystal media with polarized light hasmany attractive features. To exploit this process for use in many liquidcrystal device applications, anisotropically absorbing molecules thatabsorb in the ultraviolet (UV) region are desirable because they can betransparent in the visible region. Schadt, et al (Jpn. J. Appl. Phys.,1992, 31, 2155), for instance, has described polyvinyl cinnamates as auseful material for optical alignment of liquid crystals; and Hasegawa,et al (J. Photopolymer Sci. & Tech., 1995, 8, 241) has described UVexposure of a commercial polyimide and shown it to align liquidcrystals.

SUMMARY OF INVENTION

The instant invention provides a process for aligning a liquid crystalmedium that is useful in aligning liquid crystal displays and otherliquid crystal devices. New materials for optical alignment layers arealso disclosed that provide excellent alignment properties upon exposureto UV light.

Specifically, the present invention provides a process for aligningliquid crystal adjacent to a surface of an optical alignment layercomprising: (a) exposing at least one optical alignment layer,comprising anisotropically absorbing molecules, to polarized light; thepolarized light having a wavelength within the absorption band of saidanisotropically absorbing molecules; wherein the exposed anisotropicallyabsorbing molecules induce alignment of a liquid crystal medium at anangle + and −θ with respect to the direction of the polarization of theincident light beam and along the surface of the optical alignmentlayer; and (b) applying a liquid crystal medium to said opticalalignment layer; wherein the anisotropically absorbing molecules consistessentially of at least one diaryl ketone.

The invention also provides a liquid crystal optical storage medium,liquid crystal display element and a diffractive optical elementpreferably made by the process of the invention.

The invention further provides novel polyimide compositions forgenerating alignment of liquid crystals, consisting essentially of apolyimide polymer that is a copolyimide of at least one diaryl ketonetetracarboxylic dianhydride and at least three diamines, consistingessentially of at least three structural elements of the formula:

wherein Y is a divalent radical selected from the formulas:

wherein Z and Z₁ are independently selected from the group consisting of—S—, —SO₂—, —O—, —CH₂CH₂—, —CH₂—, —NR—, —C(CF₃)₂—, —C(O)— and a covalentbond, wherein R is a C₁-C₄ hydrocarbon chain; X₂ is independentlyselected from —R₃, —OR₃, —SR₃, and —N(R₄)R₃; wherein R₃ is selected fromC₁-C₃ perfluorinated alkyl chain and partially fluorinated alkyl chainand R₄ is independently selected from R₃ and H; X₃ is independentlyselected from X₂ and H; X is independently selected from the groupconsisting of H, Cl, F, and Br, and m is 1 or 0.

The invention further encompasses several other novel polyimidecompositions for generating alignment of liquid crystals.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a general liquid crystal displayelement of the present invention.

FIG. 2 illustrates the variation in director angle between plane i andi+1 for a twisted alignment state.

FIG. 3 illustrates several alignment regions with various twistedalignment states in a liquid crystal layer.

FIG. 4 illustrates several alignment regions with various birefringentalignment states in a liquid crystal layer.

FIG. 5 illustrates several alignment regions with various combinationalignment states in a liquid crystal layer.

FIG. 6 is a cross section illustrating the basic construction of anoptical storage medium.

FIG. 7 shows a system which can be used to expose coated substrates toultraviolet light.

FIG. 8 shows a system which can be used to expose coated substrates withultraviolet light from a UV lamp source.

DETAILED DESCRIPTION

As used herein, “substrate” means the supporting structure for analignment layer. A substrate can be any solid combination of layeredmaterials that provide a useful function for the final optical alignmentlayer or liquid crystal display. For example, the substrate can be anycombination of the following materials; crystalline or amorphoussilicon, glass, plastic, including polyester, polyethylene andpolyimide; quartz, indium-tin-oxide, gold, silver, silicon dioxide,polyimide, silicon monoxide, anti-reflective coatings, color filterlayers, polarizers and phase compensating films. In practice, some ofthese materials are deposited or coated onto a basis supportingstructure such as glass or plastic.

As used herein, the term “alignment layer” is the layer of material onthe surface of a substrate that controls the alignment of a liquidcrystal layer in the absence of an external field. A “conventionalalignment layer” herein refers to an alignment layer that will onlyalign a liquid crystal layer via processing other than optical means.For example, mechanically buffed polyimides, evaporated silicon dioxide,Langmuir-Blodgett films, have all been shown to align liquid crystals.

As used herein, the term “alignment of liquid crystals” means that thelong molecular axes of the liquid crystal molecules have a preferredlocal alignment direction, or director. The director is the averagedirection of an ensemble of liquid crystal molecules which can bequantified by order parameter or other measurements well known in theart. Orientational order parameters are routinely described by theequation:

S=½<3 cos²α−1>

where α is the angle between the director and the long axis of eachmolecule, the molecules being regarded as cylindrically symmetric. Thebrackets denote an average over the ensemble of molecules. Orderparameters range from 0 to 1.0. A 0 value indicates no long rangealignment of the liquid crystals is present. A value of 1.0 indicatesthe liquid crystal molecules are fully aligned along a director.Preferred order parameters resulting from the process of the instantinvention are in the range of about from 0.1 to 1.0.

“Optical alignment layer” herein refers to an alignment layer thatcontains anisotropically absorbing molecules that will induce alignmentof liquid crystals after exposure with polarized light. Opticalalignment layers may be processed by conventional means, such asmechanical rubbing, prior to or after exposure to polarized light. Theanisotropically absorbing molecules of the optical alignment layersexhibit absorption properties with different values when measured alongaxes indifference directions. The anisotropic absorbing moleculesexhibit absorption bands between 150 nm and about 2000 nm. Theanistropically absorbing molecules of the optical alignment layer can becovalently bonded within a main chain polymer, they can be covalentlybonded as side groups to a main polymer chain, they can be present asnonbonded solutes in a polymer, or they can be in the adjacent liquidcrystal layer as a solute and adsorbed on the surface of a normalalignment layer to give an optical alignment layer.

Preferred optical alignment layers have absorbance maxima of about from150 to 1600 nm. More preferable optical alignment layers have absorbancemaxima of about from 150 nm to 800 nm. Most preferable optical alignmentlayers for the present invention have absorbance maxima of about from150 and 400 nm and especially about from 300 to 400 nm.

Anisotropically absorbing molecules typically used in optical alignmentlayers have carbon-carbon, carbon-nitrogen, or nitrogen-nitrogen doublebonds and are capable of undergoing cis-trans isomerization uponexposure to light within their absorption band. One surprising aspect ofthe invention described herein is that diary ketones have been found toact as efficient anisotropically absorbing molecules in the opticalprocess of aligning liquid crystals. An advantage of using diarylketones is that they exhibit strong absorption in the UV region from 250to 400 nm and generally are transparent above 400 nm.

Optical alignment layers useful in the process of the invention can havea diaryl ketone present as nonbonded solute dissolved in a polymer.These are referred to as guest-host optical alignment layers. They areprepared by coating on substrates a thin layer of organic materialcontaining the diaryl ketones molecules. Typically the diaryl ketone isdissolved in solution along with a polymeric material. The solution isthen coated on substrates using, typically, a spin casting technique.The coatings are then oven baked to remove residual solvent and performthe final cure. Specific diaryl ketones preferred in guest-host opticalalignment layers are benzophenone and substituted benzophenonederivatives such as 4,4′-bis(trifluoromethyl)benzophenone,3,4′-bis(trifluoromethyl)benzophenone, and3,3′-bis(trifluoromethyl)benzophenone and 4,4′-diaminobenzophenone.

Alternatively, optical alignment layers are prepared by coatingconventional alignment layers such as a polyimide on the substrates. Thediaryl ketone is dissolved in a liquid crystal medium to give aguest-host mixture. When the guest-host mixture containing the diarylketone is allowed to contact a conventional alignment layer, an opticalalignment layer is formed.

In still another alternate preparation technique, optical alignmentlayers can be prepared by coating conventional alignment layers such aspolyimide on the substrates and the diaryl ketone is dissolved in asolvent. The solution containing anisotropically absorbing molecules iscoated on the conventional alignment layer and the solvent evaporated togiven an optical alignment layer.

A most preferred process of the invention is wherein the diaryl ketonescomprise a polymer having a recurring structural element of formula:

wherein said structural element has a covalent linkage from Q to amember selected from the group Q, Ar, and Ar′; Q is an organic radicalwith 1 to 100 atoms; Ar and Ar′ are independently selected from thegroup of substituted and unsubstituted phenyl, fused polycyclic aromaticand heteroaromatic rings. Preferred Ar and Ar′ moieties include:

wherein X′ is independently selected from the group of monovalentorganic radical of 1 to 100 atoms, a divalent organic group of 1 to 20atoms connecting Ar and Ar′ to form a ring and a covalent bondconnecting Ar and Ar′ to form a ring; t is 0 to 4; and the polymer has amolecular weight between 600 and 5 million Daltons. Polymers comprisingdiaryl ketone groups are preferred because they have greater thermal andcompositional stability than their guest-host counterparts. Chromophorescovalently bonded to polymers tend not to sublime on thermal processingor undergo dissolution in processing with solvents.

Polymers of formula I can be of a wide variety. Polymers of formula Iwherein both covalent linkages are bonded to Q have the diaryl ketonegroup, AR—C(O)—Ar′—, as a side group and —Q— as a main polymer chain.Typical polymers within this general class of polymers that are usefulin the process of the invention are poly(methyl methacrylate),poly(methyl acrylate), poly(vinyl alcohol), and poly(styrene) copolymersthat contain the diaryl ketone moiety within one of the comonomers. Forinstance, a poly(methyl methacrylate) copolymer incorporating the diarylketone moiety can be readily prepared by copolymerization of methylmethacrylate with monomer A

The synthesis of monomer A and the copolymerization of the monomer arewell known in the art, for instance in, the review by R. S. Davidson etal., J. Photochem. & Photobiol., 1995, A 89, 75-87. Other copolymerscontaining the diaryl ketone moiety can be similarly prepared usingmonomers and procedures that are all well known in the art. Preferredloadings of the diaryl ketones comonomers in the copolymers are aboutfrom 5 to 100 mole % and more preferably the loading is about from 5 to50 mole %.

Polymers of formula I wherein one covalent linkage is to Q and onecovalent linkage is to Ar have the aryl ketone moiety built into themain polymer chain. Useful main chain polymers containing diarylketonemoieties are poly(amides), poly(imides), poly(esters), poly(carbonates)and poly(siloxanes). For instance a poly(amide) of formula I that isuseful in the process of the invention is readily prepared by thecondensation of 4,4′-diaminobenzophenone with a dicarboxylic acidchloride such as adipoyl chloride to give a polymer of structure B

Likewise, condensation of 4,4′-dihydroxybenzophenone with the same acidchloride will give a polyester of structure C

Each of these polymer families, which are readily prepared by common andwell known synthetic methods, are useful in the optical process of thisinvention.

Polymers especially useful and preferred in the optical process of thisinvention are polyimides. Polyimides are known for their excellentthermal and electrical stability properties and these properties areuseful in optical alignment layers for liquid crystal displays. Thepreparation of polyimides is described in “Polyimides”, D. Wilson, H. D.Stenzenberger, and P. M. Hergenrother Eds., Chapman and Hall, New York(1990). Typically polyimides are prepared by the condensation of oneequivalent of a diamine with one equivalent of a dianhydride in a polarsolvent to given a poly(amic acid) prepolymer intermediate. Typicalsolvents used in the condensation reaction are N-methyl-pyrrolidone(NMP), dimethylacetamide (DMAc), dimethylformamide (DMF),dimethylsulfoxide (DMSO), butyl cellosolve, ethylcarbitol,γ-butyrolactone, etc. They poly(amic acid) is typically formulated togive a 1 to 30 wt % solution. The condensation reaction is usuallyperformed between room temperature and 150° C. The prepolymer solutionis coated onto a desired substrate and thermally cured at between 180and 300° C. to complete the imidization process. Alternatively, thepoly(amic acid) prepolymer is chemically imidized by addition of adehydrating agent to form a polyimide polymer. Examples of chemicalimidization reagents are organic anhydrides such as acetic anhydride andtrifluoroacetic anhydride in combination with organic bases such astriethyl amine and pyridine. Other chemical imidization reagents areethylchloroformate and triethylamine, thionyl chloride, oxalyl chloride,acetyl chloride and dicyclohexylcarbodiimide. Chemical imidizations areperformed between room temperature and 150° C. Chemical imidizationrequires that the resulting polyimide be soluble in a solvent forfurther processing. Achieving solubility often requires polyimides to bespecially formulated for chemical imidization. The chemically imidizedpolyimide solution is coated onto a substrate and heated to removesolvent, but no high temperature cure is required. Preferred opticalalignment layers of this invention are derived from chemical imidizedpolyimide solutions.

In preparing polyimides for optical alignment layers the molar ratio ofdiamine to dianhydride usually is 1:1, but can vary between 0.8:1 to1:1.2. The preferred ratio of diamine to dianhydride is between 0.98:1and 1:1.02. Most preferred is a 1:1 ratio of diamine dianhydride.

Preferred in the process of this invention is a polyimide polymer thatis a homopolyimide or a copolyimide of at least one diarylketonetetracarboxylic dianhydride and at least one diamine, which comprises atleast one structural element of formula II:

wherein Q′ is a divalent organic radical derived from said diaminecontaining at least two carbon atoms; X is independently selected fromthe group consisting of H, Cl, F, Br, R_(1 l and R) ₁O—; wherein R₁ isindependently selected from C₁-C₃ perflourinated alkyl chain, C₁-C₃partially flourinated alkyl chain and C₁-C₈ hydrocarbon chain; Z isselected from the group consisting of —S—, —SO₂—, —O—, —CH₂CH₂—, —CH₂—,—NR—, —C(CF₃)₂—, —C(O)—, or a covalent bond, wherein R is a C₁-C₄hydrocarbon chain; and m is 1 or 0.

Polyimides of formula II can be derived from diaryl ketone dianhydrides.The most common diaryl ketone dianhydrides and the most preferred familyfor this invention are the benzophenonetetracarboxylic dianhydrideswherein m is 0. Benzophenonetetracarboxylic dianhydrides preferred arethose having the following structure:

wherein X is independently selected from the group consisting of H, Cl,F, and Br.

Preferred are 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (D1) and2,2′-dichloro-4,4′,5,5′-benzophenone tetracarboxylic dianhydride. Bothmaterials are colorless, provide reasonable solubility characteristicsto the polyimides, and provide the necessary photoactive UV chromophorein high concentration.

Specific benzophenonetetracarboxylic dianhydrides preferred in thisinvention are readily available from commercial sources or synthesis.For instance, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (D1) isavailable from Aldrich Chemical Co., Inc. (1001 W. St. Paul Ave.,Milwaukee, Wis. 53233), 2,2′-Dichloro-4,4′,5,5′-benzophenonetetracarboxylic dianhydride is available from 4-chloro-o-xylene byFriedel-Crafts acylation with oxalyl chloride to give2,2′-dichloro-4,4′,5,5′-tetramethylbenzophenone, followed by oxidationwith nitric acid and dehydration of the resulting tetracarboxylic acidas described by Falcigno, et al., J. Poly. Sci. 1992, 30, 1433.

Other diaryl keytones dianhydrides that are useful in the process of theinvention, wherein m is 1, are the polycyclic diaryl ketone dianhydridesdescribed by Pfeifer, et al., in U.S. Pat. No. 4,698,295 and herebyincorporated by reference.

Diamines from which the divalent organic radical —Q′— is derived are notparticularly limited. Specific examples include the trifluoromethylsubstituted diamines 1-7 and the lower hydrocarbon homologs 8-10 ofTable 1. Other aromatic diamines can be used such as p-phenylenediamine,m-phenylenediamine, 2,5-diaminotoluene, 2,6-diaminotoluene,4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl, diaminodiphenylmethane,diaminodiphenyl ether, 2,2-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone,diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)diphenylsulfone,2,2-bis [4-(4-aminophenoxy)phenyl]propane,2,2-bis(4-aminophenyl)hexafluoropropane and2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane; alicyclic diaminessuch as bis(4-aminocyclohexyl)methane; and aliphatic diamines such astetramethylenediamine and hexamethylene diamine. Further,diaminosilixanes such as bis)3-aminopropyl)tetramethyldisiloxane may beused. Such diamines may be used alone or in combination as a mixture oftwo or more of them. Polyimides of formula II preferrably are derivedfrom diamines wherein Q′ is a divalent radical selected from formulasIII and IV

wherein Z₁ is selected, independently, from the same group as Z; each X₁is independently selected from H, Cl, F, Br, R₁, —O—R₁, —S—R₁ and—N(R₂)—R₁; wherein R₁ is independently selected from C₁-C₃perfluorinated alkyl chain, C₁-C₃ partially fluorinated alkyl chain, andC₁-C₈ hydrocarbon chain; R₂ is independently selected from H and R₁; andn is 1 to 4. Preferably Z₁ is selected from —C(O)— and a covalent bondand X₁ is independently selected from H, —CF₃, —CH₃ and —CH₂CH₃.

Diamines especially preferred in the invention are diaminobenzophenones.Diaminobenzophenones are diaryl ketones and thus act as another sourceof photoactive species in the process. In copolyimides incorporatingboth diamino and dianhydride derivatives of diaryl ketones, a largerconcentration of active chromophore can be achieved. Preferreddiaminobenzophenones for copolyimide compositions of the instantinvention is 4,4′-diaminobenzophenone and 3,4′-diaminobenzophenone.

A wide variety of other dianhydrides, of course, may be used incombination with the diaryl ketone dianhydrides in forming copolyimidesuseful in the process of the invention. Specific examples of thetetracarboxylic dianhydride component include aromatic dianhydrides suchas pyromellitic dianhydride, 2,3,6,7-napthalenetetracarboxylicdianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride,3,3′4,4′-biphenyltetracarboxylic dianhydride,2,3,2′,3′-biphenyltetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride,bis(3,4-dicarboxyphenyl)diphenylsulfone dianhydride,bis(3,4-dicarboxyphenyl)methane dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane dianhydride(D2), bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride,2,3,4,5-pyridinetetracarboxylic dianhydride; alicyclic tetracarboxylicdianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride,1,2,3,4-cyclobutanetetracarboxylic dianhydride,1,2,3,4-cyclopentanetetracarboxylic dianhydride,1,2,4,5-cyclohexanetetracarboxylic dianhydride,2,3,5=tricarboxycyclopentylacetic acid dianhydride and3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalenesuccinic dianhydride; andtheir acid and acid chloride derivatives.

“Alicyclic tetracarboxylic dianhydrides” refer to dianhydrides that arecomprised either partially or in whole of saturated carbocyclic rings.The alicyclic tetracarboxylic dianhydrides impart useful solubilityproperties to polyimides comprising them. Alicyclic tetracarboxylicdianhydrides suitable for the invention are those listed in Table 2.

5-(2,5-Dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride (D3) is commercially available from Chriskev Co, Inc.2,3,5-Tricarboxycyclopentaneacetic acid dianhydride (D4) is availablevia synthesis by oxidation of dicyclopentadiene with nitric acid asdescribed by Hession, et al., in British Patent 1 518 322 (1976). Thesynthesis of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (D5) isdescribed by Moore, et al, Chem. Mat., 1989, /, 163,1,2,3,4-butanetetracarboxylic dianhydride (D7) is available bydehydration of the tetracarboxylic acid (Aldrich Chemical) with aceticanhydride.5,5′-(1,1,3,3-Tetramethyl-1,3-disiloxanediyl)-bis-(norbornane-2,3-dicarboxylicanhydride) (D8) is available by hydrosilation of5-norbornene-2,3-dicarboxylic anhydride with1,1,3,3-tetramethyldisiloxane as described by Ryang in U.S. Pat. No.4,381,396. Bicyclo[2.2.1]heptanetetracarboxylic 2,3:5,6-dianhydride (D9)is available by synthesis from bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylicanhydride as described by Matsumoto et al., in Macromolecules 1995, 28,5684. Bicyclo[2.2.2]oct-7-enetetracarboxylic 2,3:5,6-dianhydride (D10)is available by synthesis from 4-cyclohexene-1,2-dicarboxylic anhydrideas described by Itamura. et al., in Macromolecules 1993, 26, 3490.

The addition of alicyclic dianhydride structural units to polyimideformulations tends to generate soluble polyimides and thus, impart veryuseful processing properties to the polyimides. For instance, chemicalimidization may be performed without the resulting polyimide becominginsoluble. Thus, in a preferred process of the invention the polyimideis a copolyimide that includes structural elements of formula II andformula V

wherein P is a tetravalent organic radical derived from said alicyclictetracarboxylic dianhydride and Q′ is as described above. Preferredalicyclic dianhydrides in this process are5-(2,5-dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride (D3), 2,3,5-tricarboxycyclopentaneacetic acid dianhydride(D4), 1,2,3,4-butanetetracarboxylic dianhydride (D7) and1,2,3,4-cyclobutanetetracarboxylic dianhydride (D5).

A dianhydride that is especially useful and preferred in combinationwith alicyclic dianhydrides is1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane dianhydride(D2). This dianhydride, when present in the polyimide, tends to improvethe quality of alignment of liquid crystals in comparison to thosepolyimides formulated from only alicyclic dianhydrides.

Preferred processes are those wherein the tetracarboxylic dianhydridefrom which structural formula II is derived3,3′,4,4′-benzophenonetetracarboxylic dianhydride, the alicyclictetracarboxylic dianhydride from which structural formula V is derivedis 5-(2,5-dioxotetrahydro) -3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride and Q′ is derived from at least one diamine selected from thegroup consisting of 2-(trifluoromethyl)-1,4-benzenediamine,5-(trifluoromethyl)-1,3-benzenediamine,2,2′-bis(trifluoromethyl)benzidene, 3,3′-bis(trifluoromethyl)benzideneand 4,4′-diaminobenzophenone.

A further embodiment of the invention is a process wherein the polyimidepolymer is a homopolyimide or a copolyimide of at least onetetracarboxylic dianhydride and at least one diaminobenzophenone, whichcomprises at least one structural element of formula VI

wherein M is a tetravalent organic radical derived from saidtetracarboxylic dianhydride containing at least two carbon atoms, nomore than two carbonyl groups of the dianhydride being attached to anyone carbon atom of the tetravalent radical; and X is as described above.A preferred diaminobenzophenone is 4,4′-diaminobenzophenone and apreferred tetracarboxylic dianhydride is an alicyclic tetracarboxylicdianhydride as discussed above.

Other embodiments of the instant invention are novel compositions forgenerating alignment of liquid crystals. One such embodiment encompassesa novel composition consisting essentially of a polymide polymer that isa copolyimide of at least one diaryl ketone tetracarboxylic dianhydrideand at least three diamines, consisting essentially of at least threestructural elements of the formula IIa

wherein Y is a divalent radical selected from the formulas IIIa and IVa

wherein Z and Z₁ are selected, independently, from the group consistingof —S—, —SO₂—, —O—, —CH₂CH₂—, —CH₂—, —NR—, —C(CF₃)₂—, —C(O)— and acovalent bond, wherein R is a C₁-C₄ hydrocarbon chain; X₂ isindependently selected from —R₃, —OR₃, —SR₃, —N(R₄)R₃, wherein R₃ isselected from C₁-C₃ perfluorinated alkyl chain and partially fluorinatedalkyl chain and R₄ is independently selected from R₃ and H; X₃ isindependently from X₂ and H; X is independently selected from the groupconsisting of H, Cl, F, and Br; and m is 1 or 0.

Another preferred composition for generating alignment of liquidcrystals consists essentially of a polyimide polymer that is acopolyimide of at least one diaryl ketone tetracarboxylic dianhydrideand at least two diamines, consistings essentially of at least onestructural element of the formula IIa and at least one structuralelement of formula VII

wherein Z, Z₁, X, X₂, X₃, and m are as previously described and thecopolyimide consists of 99 to 1 mol % of at least one structural elementof formula IIa and 1 to 99 mol % of at least one structural element offormula VII.

Another preferred novel composition is a copolyimide of at least onediaryl ketone tetracarboxylic dianhydride, at least one alicyclictetracarboxylic dianhydride and at least one diamine, consistingessentially of at least two structural elements of the formula IIa andVa, wherein the copolyimide comprises about from 95 to 20 mol % of atleast one structural element of formula IIa and about from 5 to 80 mol %of at least one structural element of formula Va.

Another preferred novel composition consists essentially of a polyimidepolymer that is a homopolyimide or copolyimide of at least onediaminobenzophenone and at least one alicyclic tetracarboxylicdianhydride, consisting essentially of at least one structural elementof the formula VIII

wherein P and X are as described above.

Another preferred novel composition is a copolyimide of at least onediaminobenzophenone, at least one aclicyclic tetracarboxylic dianhydrideand at least one diaryl ketone tetracarboxylic anhydride, consistingessentially of at least one structural element of formula VIII and onestructural element VII

wherein X, Z and m are as described above. In a more preferredcomposition the copolyimide comprises about from 5 to 99.9 mol % of atleast one structural element of formula VIII and about from 95 to 0.1mol % of at least one structural element of formula VII.

Another preferred novel composition is a copolyimide of at least onediaminobenzophenone, at least one alicyclic tetracarboxylic dianhydrideand 1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propanedianhydride, consisting essentially of at least one structural elementof formula VIII and at least one structural element of formula IX

wherein X is as described above. In a more preferred composition thecopolyimide consists essentially of about from 0.1 to 99 mol % of atleast one structural element of formula IX and about from 99.9 to 1 mol% of at least one structural element of formula VIII.

The novel optical alignment layers derived from polyimides of formulaIIa, upon exposure to polarized UV light, exhibit exceptionally highquality alignment of liquid crystals.

Preferred diamines for the novel compositions of the instant inventionare those in which X₁ is —CF₃ or —OCF₃. Specific diamines that are morepreferred are 2-(trifluoromethyl)-1,4-benzenediamine (1),5-(trifluoromethyl)-1,3-benzenediamine (2),2,2′-bis(trifluoromethyl)benzidene (5),2,2′-bis(trifluoromethoxy)benzidene (6) and3,3′-bis(trifluoromethyl)benzidene (7). Most preferred diamines are2-(trifluoromethyl)-1,4-benzenediamine and2,2′-bis(trifluoromethyl)benzidene Furthermore, when at least threedifferent fluorinated diamines derived from formula IIIa and IVa arepresent the polyimide tends to maintain solubility after chemicalimidization, and thus thermal processing of the optical alignment layercan be minimized.

Specific diamines useful in this invention are readily available fromcommercial sources. For instance, 2-(trifluoromethyl)-1,4-benzenediamine(1) and 5-(trifluoromethyl)-1,3-benzenediamine (2) are available fromPCR Inc. (P.O. Box 1466, Gainesville, Fla., 32602); and2,2′-bis(trifluoromethyl)benzidene (5) is available for Chriskev Co.,Inc. (5109 W. 111th Tr., Leawood Kans.).2,2′-Bis(trifluoromethoxy)benzidene (6) is prepared by reduction of3-(trifluoromethoxy)nitrobenzene to the corresponding hydrazoderivative, followed by benzidine rearrangement as described by Feiring,et al., Macromolecules 1993, 26 2779. 3,3′-Bis(trifluoromethyl)benzidene(7) is available via the Ulman coupling of3-bromo-6-nitrobenzotrifluoride as described for the 2,2′-isomer inGaudiana, et al., J. Polym. Sci., Part A 1987,25 1249-1271) followed bychemical reduction of the dinitro compound to the diamine with tin (II)chloride in ethanol.

The novel compositions of the instant invention are useful in alignmentof liquid crystals. Upon exposure to polarized UV light these materialsexhibit fair to excellent quality alignment of a variety of liquidcrystals. Furthermore, many of the compositions show excellent qualityalignment of liquid crystals upon mechanical buffing or rubbing of thealignment layer with a fiberous cloth.

Compositions consisting of three or more diamines derived from radicalIIIa and IVa, and compositions of alicyclic dianhydrides exhibit goodsolubility in typical solvents used in polyimide condensation. In thesecases, the poly(amic acid) intermediates can be chemically imidized togive soluble polyimide solutions. These pre-imidized solutions arepreferred for preparing optical alignment layers because they requireonly low temperature cure

Preferred alicyclic tetracarboxylic dianhydrides for compositions of theinstant invention are 5-(2,5-dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 2,3,5-tricarboxycyclopentaneacetic aciddianhydride, 1,2,3,4-butanetetracarboxylic dianhydride, and1,2,3,4-cyclobutanetetracarboxylic dianhydride. Especially preferredalicyclic tetracarboxylic dianhydrides are for compositions of theinstant invention are5-(2,5-dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride and 1,2,3,4-butanetetracarboxylic dianhydride.

To prepare the optical alignment layers of this invention poly(amicacid) solutions or preimidized polyimide solutions polymer solutions arecoated onto desired substrates. Coatings is usually accomplished with 2to 30 wt % solids. Any conventional method may be used to coat thesubstrates including brushing, spraying, spin-casting, dipping orprinting. The coated substrates are heated in an oven under an inertatmosphere, for instance nitrogen or argon, at elevated temperatureusually not exceeding 300° C. and preferably at or below 180° C. forabout from 1 to 12 hours, preferably for about 2 or less. The heatingprocess removes the solvent carrier and may be used to further cure thepolymer. For instance, the poly(amic) acid films are thermally cured togenerate polyimide films.

The optical alignment layers are exposed to polarized light to inducealignment of liquid crystals. By “polarized light” is meant light thatis elliptically polarized such that the light is more polarized alongone axis (referred to as the major axis) versus the orthogonal axis(referred to as the minor axis). The preferred polarization is linearlypolarized light where the light is polarized mostly along one axis (themajor axis) with little or no polarization component along the minoraxis. In this invention the polarized light has one or more wavelenghtsof about from 150 to 2000 nm and preferably of about from 150 to 1600 nmand more preferably about from 150 to 800 nm. Most preferably, thepolarized light has one or more wavelengths of about from 150 to 400 nm,and especially about from 300 to 400 nm. A preferred source of light isa laser, e.g., an argon, helium neon, or helium cadmium. Other preferredsources of light are mercury are deuterium and quartz tungsten halogenlamps, xenon lamps and black lights in combination with a polarizer.Polarizers useful in generating polarized light from nonpolarized lightsources are interference polarizers made from dielectric stacks,absorptive polarizers and reflective polarizers based on Brewsterreflection. With lower power lasers or when aligning small alignmentregions, it may be necessary to focus the light beam onto the opticalalignment layer.

By “exposing” is meant that polarized light is applied to the entireoptical alignment layer or to a portion thereof. The light beam may bestationary or rotated. Exposures can be in one step, in bursts, inscanning mode or by other methods. Exposure times vary widely with thematerials used, etc., and can range from less than 1 msec to over anhour. Exposure may be conducted before or after contacting the opticalalignment layer with the liquid crystal medium. Exposing can beaccomplished by linearly polarized light transmitted through at leastone mask having a pattern or with a beam of linearly polarized lightscanned in a pattern. Exposing also may be accomplished usinginterference of coherent optical beams forming patterns, i.e.,alternating dark and bright lines.

Exposure energy requirements vary with the formulation and processing ofthe optical alignment layer prior and during exposure. For example,materials that possess high glass transition temperatures can havehigher energy density requirements for optical alignment. Whereas,material systems designed to have a low glass transition temperatureprior to exposure can have lower energy density requirements. Apreferred range of exposure energy is about from 0.001 to 2000 J/cm².More preferred is the range of about from 0.001 to 100 J/cm² and mostpreferred range of exposure energy is about from 0.001 to 5 J/cm². Lowerexposure energy is most useful in large scale manufacturing of opticalalignment layers and liquid crystal display elements. Lower exposureenergy also minimizes the risk of damage to other materials on thesubstrates.

The efficiency of the alignment process, and the exposure energyrequired, may be further impacted by heating, beyond that inherent inthe “exposing” step. Additional heating during the exposing step may beaccomplished by conduction, convection or radiant heating, or byexposure to unpolarized light. Additional heating may increase themobility of the molecules during exposure and improve the alignmentquality of the optical alignment layer. Additional heating is not arequirement of the process of the invention but may give beneficialresults.

Exposing also can consist of two or more exposure steps wherein theconditions of each step such as angle of incidence, polarization state,energy density, and wavelength are changed. At least one of the stepsmust consist of exposure with linearly polarized light. Exposures canalso be localized to regions much smaller than the substrate size tosizes comparable to the entire substrate size. A preferred method ofdual exposing comprises a two step process of:

(a) exposing at least one optical alignment layer to polarized light ata normal incidence, and

(b) exposing the optical alignment layer to polarized light at anoblique incidence.

Another preferred method of dual exposing comprises a two step processof:

(a) exposing said optical alignment layer to polarized light of a firstdirection of linear polarization of the incident light and

(b) exposing said optical alignment layer to polarized light of a seconddirection of linear polarization of the incident light.

Another preferred method of dual exposing comprises a two step processof:

(a) exposing said optical alignment layer to polarized light of a firstdirection of linear polarization of the incident light, and

(b) exposing said optical alignment layer to polarized light of a seconddirection of linear polarization of the incident light, at an obliqueincidence.

Applying a liquid crystal medium to the optical alignment can beaccomplished by capillary filling of a cell, by casting of a liquidcrystal medium onto an optical alignment layer, by laminating apreformed liquid crystal film onto an optical alignment layer or byother methods. Preferred methods are capillary filling of a cell andcasting of a liquid crystal medium onto an optical alignment layer.Optical alignment layers are pre-exposed to polarized light or the areexposed after contacting the liquid crystal medium.

A cell can be prepared by using two coated substrates to provide asandwiched layer of liquid crystal medium. The pair of substrates canboth contain optical alignment layers or a conventional alignment layer(e.g., mechanically buffed) can be used as the second alignment layercomprising the same or a different polymer.

As liquid crystal substances used for liquid crystal optical elements,nematic liquid crystal substances, ferroelectric liquid crystalsubstances, etc. are usable. Useful liquid crystals for the inventiondescribed herein include those described in U.S. Pat. No. 5,032,009 andnew superfluorinated liquid crystals exemplified by ZLI-5079, ZLI-5080,ZLI-5081, ZLI-5092, ZLI-4792, ZLI-1828, MLC-2016, MLC-2019, MLC-6252,and MLC-6043 available from EM Industries, Hawthorne, N.Y. Also usefulare guest-host formulations prepared with all types of liquid crystalsand anisotropically absorbing dyes as described in U.S. Pat. No.5,032,009. Also useful in this invention are nematic and ferroelectricliquid crystals that are disclosed in U.S. Pat. No. 5,447,759 entitled“Liquid Crystal Alignment Film and Liquid Crystal Display Elements,”hereby incorporated by reference.

Chiral dopants are often added to these liquid crystals to induce atwist in one direction, in the liquid crystal medium. Left and righthanded chiral dopants are available. Typical examples are ZLI-811,S-1011 and R-1011, all available from EM Industries.

Other liquid crystals useful in this invention include the polymerizableliquid crystals as described in U.S. Pat. No. 5,073,294 and the liquidcrystal difunctional methacrylate and acrylate monomers as described inU.S. Pat. No. 4,892,392. Both patents are hereby incorporated byreference.

Still other liquid crystals useful in this invention include the liquidcrystal polymers described in U.S. Pat. No. 5,382,548 which is herebyincorporated by reference. These polyester and polyurethane liquidcrystal polymers have low rotational viscosities between their glasstransition (T_(g)) and their isotropic transition (T_(ni)) and readilyrespond to surface aligning forces.

Preferred liquid crystals for the invention are nematic liquid crystals,ferroelectric liquid crystals, polymerizable nematic liquid crystals andnematic liquid crystalline polymers. Especially preferred liquid crystalfor the invention are nematic liquid crystal and polymerizable nematicliquid crystals. Specific families of nematic liquid crystals that arepreferred are the 4-cyano-4′-alkylbiphenyls,4-alkyl-(4′-cyanophenyl)cyclohexanes and the superfluorinated liquidcrystal mixtures selected from the group consisting of ZLI-5079,ZLI-5080, ZLI-5081, ZLI-5092, ZLI-4792, ZLI-1828, MLC-2016, MLC-2019,MLC-6252, and MLC-6043 commercially available from EM Industries,Hawthorne, N.Y.

The exposed optical alignment layer induces alignment of a liquidcrystal medium at an angle + and −0 with respect to the direction of thelinear polarization of the incident light beam and along the plane ofthe optical alignment layer. One skilled in the art will recognize thatthe process of the instant invention allows control of the alignment ofa liquid crystal medium in any desired direction within the plane of theoptical alignment layer by controlling the conditions of the polarizedlight exposure. Preferrably the liquid crystal medium is aligned at anangle + and −0, where θ is equal to about 90° to the polarizationdirection.

An important feature of this invention is that after the process iscompleted the liquid crystal medium has “memory”, i.e., will maintainthe alignment which is induced by the linear polarization of theincident light source. The liquid crystal medium also can be realignedto the original or a third alignment by the process of this invention.

The alignment characteristics attainable with various formulations andthe process of the invention are described in the accompanying examples.One interesting and useful feature of the alignment process of theinvention is that, in general, very low pre-tilt angles, usually within0 to 0.3 degrees are observed. With mechanically rubbed alignment layersthe liquid crystal molecules in contact with the alignment layer alignparallel to the buffing direction, but usually not exactly parallel tothe substrate. The liquid crystal director is slightly titled from thesubstrate, for instance by about 2-10 degrees. Thus, in liquid crystaldisplay applications where a minimum pre-tilt is desired the process ofthis invention becomes especially useful.

The process of the invention and the novel optical alignment layers ofthis invention can be used to make a novel liquid crystal opticalelements, also of this invention. The novel optical elements includehuman and machine readable liquid crystal display elements,electro-optical light modulators, all optical light modulators, erasableread/write optical data storage media, image storage media, anddiffractive optical elements, both passive and active, includinggratings, beamsplitters, lenses, Fourier processors, optical disc andlaser diode radiation collimators. Several of these optical elements aredescribed in greater detail in U.S. Pat. No. 5,032,009, in columns 7 and8 and in examples 19-21, which is hereby incorporated by reference.Detailed description of a liquid crystal display element and an opticaldata storage medium of the invention follow herein.

DISPLAY ELEMENT

A liquid crystal display element made by the process of the instantinvention is composed of an electrode substrate having at least oneoptical alignment layer, a voltage-impressing means and a liquid crystalmaterial. FIG. 1 illustrates a typical liquid crystal display element,comprising a transparent electrode 2 of ITO (indium-tin-oxide) or tinoxide on a substrate 1 and optical alignment layers 3 formed thereon.The optical alignment layers are exposed to polarized light of awavelength or wavelengths within the absorption band of theanisotropically absorbing molecules. A spacer concurrently with asealing resin 4 is intervened between a pair of optical alignment layers3. A liquid crystal 5 is applied by capillary filling of the cell andthe cell is sealed to construct a liquid crystal display element.Substrate 1 may comprise an overcoat film such as an insulating film, acolor filter, a color filter overcoat, a laminated polarizing film etc.These coatings and films are all considered part of the substrate 1.Further, active elements such as thin film transistors, a nonlinearresistant element, etc. may also be formed on the substrate 1. Theseelectrodes, undercoats, overcoats, etc. are conventional constituentsfor liquid crystal display elements and are usable in the displayelements of this invention. Using the thus formed electrode substrate, aliquid crystal display cell is prepared, and a liquid crystal substrateis filled in the space of the cell, to prepare a liquid crystal displayelement in combination with a voltage-impressing means.

OPTICAL DATA STORAGE MEDIUM

An optical data storage medium with gray scale capability can beprepared by the process of the invention. A construction of an opticaldata storage medium is illustrated in FIG. 6 and discussed furtherbelow. To understand the storage medium construction several elementsare defined as follows:

“Alignment layer pairs” herein refers to two alignment layers thatcontrol the alignment of the same liquid crystal layer.

“Alignment region” refers to a continuous area of a liquid crystal layerthat has the same alignment state. The alignment region can be between0.01 and 10⁶ μm² (μm equals micrometer). Preferred alignment regionsrange in size from 0.1 to 10⁶ μm². Most preferred alignment regionsrange in size from 0.1 to 100 μm². In the liquid crystal technology,alignment regions as defined herein are often referred to as “domains”.However, in information storage technology a domain is used to describeany uniform area (bubble, colored spot, reflective surface, etc.) thatdefines an information bit. Herein alignment region will be used asdefined above and domain will be used to describe uniform areas instorage media other than liquid crystal media.

“Alignment states” refers to three distinct types of alignment:birefringent alignment, twist alignment, and combination alignment. Eachtype of alignment possesses the capability for three or more discreet,distinguishable states. Each alignment region within a liquid crystallayer of this invention takes on a type of alignment and a discreetalignment state. All alignment regions do not have to possess uniquealignment states. The same alignment state may occur many times indifferent alignment regions through a liquid crystal layer.

By a “twist alignment state” is meant the alignment regions differ by achange in twist. By “twist” or “twisted alignment” is meant that thedirection of the local alignment of the liquid crystal layer between analignment layer pair varies in a continuous fashion from one alignmentlayer to the other. As illustrated in FIG. 2, the projected directorangle in the plane of alignment layer I, γ_(i), is different from theprojected director angle in the plane of alignment layer i+1, γ_(i+1),and, as a result, the local liquid crystal projected directorcontinuously changes form γ_(i) to γ_(i+1) creating a twist structure inthe liquid crystal layer. For this invention, the twist angle,γ₁=γi+1_(−i), can vary from −360 degrees to 360 degrees. To get greaterthan a 90° or less than −90° twist angle, a chiral dopant such as CB-15(EM Chemicals, Hawthorne, N.Y.) is incorporated.

When the twisted angle, γ₁, is equal to zero, there is not twist in theliquid crystal medium and the medium is said to be parallel aligned.Most liquid crystal display applications currently use γ₁ equal to 90 or−90 degrees.

In a twisted alignment state each alignment region can have a twistother than zero and the twist value varies from region to region. It isfurther stipulated that the projection of the local liquid crystaldirector onto one alignment layer is in the same direction for alldomains at that alignment layer, whereas the projection of the localliquid crystal director on the second alignment layer varies indirection for each alignment region to create the variation in twistvalue. FIG. 3 illustrates a liquid crystal layer having severalalignment regions with variations in twist value. The solid lines withineach alignment region at the planes i and i+1 indicate the director ofthe liquid crystal at that plane. The dashed lines indicate how thedirector rotates in proceeding from one plane to the other. For instanceregion t₁ has a 180° twist value, t₂ has a 0° twist value and t₃ has a270° twist value.

FIG. 3 is a composite mean to demonstrate the wide variation in twistangles available by control of the optical alignment layer. As mentionedabove, to get greater than a 90° twist or less than a −90° twist achiral dopant is required to induce a twisted nematic structure in theliquid crystal medium. In the normal case where a uniform concentrationof chiral dopant is present the liquid crystal medium will have auniform pitch. This pitch will determine the range, within which, thevariation in twist angle induced by the optical alignment process willoccur. If the uniform concentration of chiral dopant is present whichgives a uniform twist angle of γ₁, then the range of twist variationthat can be controlled optically is γ₁±90°. For example, if the chiraldopant concentration is chosen to give γ₁=270° then the range ofoptically controlled twist variation would be 180° to 360°.

By a “birefringent alignment state” is meant the alignment regions in aliquid crystal layer differ by a change in birefringence. Each alignmentregion has zero twist (γ₁=0 degrees) but the projection of the liquidcrystal director onto the alignment layer varies in direction for eachalignment region. A liquid crystal layer with several birefringentalignment states is illustrated in FIG. 4. Alignment regions b₁, b₂ andb₃ have various directors of the local alignment relative to thebackground alignment in plane, i, 60°, 0° and 90°, respectively.

By a “combination alignment state” is meant that one or more alignmentregion in the liquid crystal layer differs by a change in twist andbirefringence. Thus, for each alignment region, the twist can vary inmagnitude and the projection of the local liquid crystal director ontoeach alignment layer can vary in direction. FIG. 5 illustrates a liquidcrystal alignment layer with several alignment regions with differentcombination alignment states. The projection of the local liquid crystaldirector onto both alignment layers may be different. Alignment regionsc₁, c₂ and c₃ all have different twist values and different directors ofthe local alignment. For instance, c₁ has a 45° twist and a 90° changein director relative to the background alignment of i; c₂ has a 0° twistand a 0° change in director; and c₃ has a 270° twist and a 0° change indirector. Again, the range of the twist angle for each liquid crystallayer depends upon the pitch of the liquid crystal medium.

By “grayscale” is meant that each domain in the optical storage mediumcan be encoded with three or more values. For example, if each domaincan be encoded with N possible values (where N is an integer), then eachdomain must have N distinct states that are measured by the detectionsystem suitable for the particular application. The size of N isdetermined by the sensitivity of the optical storage medium and/or thesensitivity of the detection system.

Traditionally, the N possible values are represented as a power of two.If there are N=16 possible values, the 16 values (thus 16 distinctstates for each domain in the media) would be represented as 2⁴ andcalled 4 bit grayscale. The term bit represents the power of tworequired to get the total possible values. For example, 0 bit wouldrepresent 1 possible encoded value, 1 bit=2 possible encoded values, . .. j bit=2^(j) possible encoded values.

In most applications of optical storage media, the distinct states ofeach domain cause a variation in the transmitted or reflected flux oflight incident onto a light sensitive detector. For each distinct statethere is a distinct light level transmitted or reflected. Solid statedetectors based on semiconductors, and the human eye are a few examplesof light sensitive detectors. Each detection system must process thetransmitted or reflected light signals into information useful forinterpretation.

In the case of solid state detectors, electronic processing converts thelight flux level to a binary number (power of two) that is interpretedby a computer. The computer processes the binary number from each domaineither serially (one domain at a time) or in parallel (multiple domainsat a time) and derives the information it needs to perform theprogrammed task.

In the case of the human eye, the distinct light levels for each domainare processed in parallel by the brain to create a photographic image.Each domain by itself is not very meaningful to the brain but the sumtotal of all the transmitted or reflected light levels from each domainresult in meaningful information that is interpreted by the brain.

Most optical storage mediums demonstrate only one bit of information canbe recorded in a single domain. As a consequence, only two distinctstates are possible (i.e., a 0 or 1 in a binary number system).Therefore, to store the number 16 in this medium, four domains would beneeded. However, if each domain had 16 distinct states then a singledomain could be used to store the same number that required four domainsin the 1 bit medium. Thus, we have effectively increased the storagedensity by four times. Carrying this argument to its logical conclusion,if 2^(k) detectable states were available for each domain, the storagedensity of the medium would increase by k times.

In the context of the present invention, each liquid crystal alignmentregion in the optical storage medium can be encoded with a distinctbirefringent alignment state, twist alignment state, or a combinationalignment state. If polarized light is transmitted or reflected throughthe medium and then passed through a polarizer the distinct alignmentstate would change the light level detected. For example, if the twistalignment state is used for each alignment region, there is a distinctlight level detected for twist alignment states with their twist valuelimited to one quadrant (0 to 90 degrees, 90 to 180 degrees, etc.) of afull 360 degree twist. For one quadrant and N distinct twist alignmentstates, there are 90/N possible light levels to be detected. Thereforeeach alignment region in the medium of the present invention is capableof grayscale. Similar arguments can be made for the birefringent andcombination alignment states used in the present invention.

To obtain the desirable features of gray scale in a medium, greater thantwo alignment states are required. A preferred optical storage medium ofthe invention described herein has been 4 and 2000 alignment states.

FIG. 6 is a cross section illustrating the basic construction of anoptical storage medium prepared by the process of the instant invention.A series of substrates 1 are coated with alignment layers 3 on one orboth sides. The coated substrates then stacked in series and spacedappropriately with spacers (not shown). The series of substrates may besealed at the perimeter (not shown), except for fill and exit ports,with a sealing compound to make a cell. The cell is then filled with adesired liquid crystal to provide liquid crystal layers 5, followed bysealing the fill and exit ports. Each additional dual coated substratein the series allows for a repetition 6 comprising an additional liquidcrystal layer. Repetitions may number from X=0 to about 20 and are onlylimited by the capability to address each liquid crystal layer.

As can be seen from FIG. 6, each liquid crystal layer has acorresponding alignment layers pair that controls the alignment of theliquid crystal layer. In this invention at least one of the alignmentlayers of each alignment layer pair is an optical alignment layer.Different alignment regions 7 of the liquid crystal layer have three ormore alignment states that result in grayscale. In FIG. 6 the specificalignment states of the alignment regions 7 may be different twisted,birefringent, or combination alignment states, or a mixture of all threetypes of states, illustrated in FIGS. 3-5. The alignment states arecontrolled by exposure of selected regions of the optical alignmentlayer with polarized light. Each liquid crystal layer can be selectivelyaddressed by matching the absorption characteristics of thecorresponding optical alignment layer with the appropriate wavelength ofpolarized light. Exposure of the optical alignment layer with polarizedlight can be accomplished before or after construction of the cell andbefore or after contact with the liquid crystal medium.

Exposure of selected regions of one optical alignment layer, of analignment layer pair, to polarized light while the other alignment layerremains fixed allows formation of twisted alignment states. Equalexposure of selected regions of two optical alignment layers, of analignment layer pair, to polarized light gives a birefringent alignmentstate. When selected regions of two optical alignment layers of analignment layer pair are differently exposed to polarized light, acombination alignment state is created.

This invention is demonstrated in the following examples, which areillustrative and not intended to be limiting.

EXAMPLE 1

This example illustrates the optical process for aligning liquidcrystals using a homopolyimide of one structural element of formula IIa.

A mixture of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, D1, (322mg, 1.0 mmol, 98 wt %), 2-trifluoromethyl)-1,4-benzenediamine, 1, (176mg, 1.0 mmol) and γ-butyrolactone (7.0 mL) was stirred at roomtemperature for 16 h under a nitrogen atmosphere. The solution wasdiluted to a 3 wt % solution by addition of γ-butyrolactone (7.4 mL),filtered through a 0.45 μm teflon filter and spin coated onto soda-limeglass substrates (0.9″×1.2″) at 2500 rpms. The coated substrates weredried at 80° C. for 0.25 h and 180° C. for 1 h in a nitrogen atmosphereand stored in a nitrogen atmosphere at room temperature until used.

The coated substrates were exposed to ultraviolet polarized light usingthe set-up schematically represented in FIG. 7. In this experiment eachcoated substrate 8 was mounted onto a 2-axis XY translation stage(indicated by double-headed arrows 9 in FIG. 7) with the coated sidefacing the incident laser beam. An Innova 400 (Coherent Incorporated,Santa Clara, Calif.) laser 10 was tuned to laser in the ultraviolet withwavelengths ranging from 308 to 336 nm. The 1 cm polarized beam 11 wasdirected with mirror 12 to a 5 cm focal length cylindrical lens 13 whichfocused the incident 1 cm beam to a line (1 cm×200 μm) onto each coatedsubstrate 8. The coated substrate was translated at a 0.5 mm/s constantspeed along the Y direction and then stepped in the X direction. Thiswas repeated until the coated substrate had been completely exposed. Theincident optical power was 0.25 Watts and the ultraviolet light waspolarized along 11.

A liquid crystal cell was constructed from the two exposed coatedsubstrates. Mylar polyester strips (55 μm) were placed on one coatedsubstrate and the other substrate was sandwiched on top of it. Theoptical alignment layers were facing each other and the backgroundalignment directions were mutually parallel. The substrates were pressedto a 55 micrometer spacing using clamps, epoxy was applied along theedges and the epoxy was cured for 5 mins. Two spaces on opposite edgesof the cell were left unsealed. One unsealed opening on the cell wasdipped into MLC6043 nematic liquid crystal (EM Industries, Inc.,Hawthorne, N.Y.). The cell filled by capillary action. After filling,the cell was removed from the liquid crystal, cleaned up, and the spacessealed with epoxy.

The cell was viewed between crossed polarizers on a photographic lightbox. The background alignment direction was along one of the inputpolarizer's transmission axis. The input polarizer polarizes the lightalong the background alignment. The output polarizer blocks thetransmission of the light since its transmission axis is crossed to theinput polarizer's transmission axis. As a result, the background of thecell appeared uniformly dark except for occasional disclination linesindicated that the exposure of the coated substances induced the netuniform alignment of the liquid crystal sandwiched between the opticalalignment layers.

In another trail two optical alignment layers were treated identicallyas described above except that the coated substrates were translated ata 1.5 mm/s constant speed along the Y direction and then stepped in theX direction (e.i. higher can rate). The same results between crossedpolarizers were observed as described above.

In another trial two optical alignment layers were treated identicallyas described in the first trial except that nitrogen gas was blownacross the substrates during exposure to reduce the oxygen levels toless than 0.3% (measured using Sensidyne Oxytec Mini Monitor, ModelGOA-2H, Clearwater, Fla.). The same results between crossed polarizerswere observed as as described above. Thus, exposure under a nitrogenpurge had no effect on the outcome of the trial.

EXAMPLE 2

This example further illustrates the optical process for aligning liquidcrystals using a homopolyimide of one structural element of formula VII.

A mixture of dianhydride D1 (161.1 mg, 0.50 mmol, 98 wt %),4,4′-diaminobenzophenone (106.1 mg, 0.50 mmol, Aldrich) andγ-butyrolactone (2.40 g) was stirred at room temperature for 16 h undera nitrogen atmosphere. The solution was diluted to a 3 wt % solution byaddition of γ-butyrolactone (6.23 g), filtered through a 0.45 μm teflonfilter and spin coated onto soda-lime glass substrates (0.9″×1.2″) at2500 rpms. The coated substrates were dried at 80° C. for 0.25 h and180° C. for 1 h in a nitrogen atmosphere and stored in a nitrogenatmosphere at room temperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was repeated except the scan rate was chosen to be 1.5mm/sec. The same results between crossed polarizers were observed asdescribed above.

EXAMPLE 3

This example illustrates the formation and use of novel compositions ofthis invention consisting essentially of copolyimide with three or morediamines derived from formulas IIa.

A mixture of dianhydride D1 (161.1 mg, 0.05 mmol), diamines 1 (44.0 mg,0.25 mmol), 2 (22.0 mg, 0.125 mmol), 5 (40.0 mg, 0.125 mmol) andγ-butyrolactone (3.28 g) was stirred 16 h at room temperature under anitrogen atmosphere. Triethylamine (0.35 mL, 2.5 mmol) and a solution ofacetic anhydride (0.142 mL, 1.5 mmol) in γ-butyrolactone (0.9 g) wereadded sequentially, and the mixture was heated to 120° C. for 2.25 h.The solution was cooled to room temperature, diluted to a 3 wt %solution by addition of γ-butyrolactone (4.04 g), filtered through a0.45 μm teflon filter and spin coated onto soda-lime glass substrates(0.9″×1.2″) at 2500 rpms. The coated substrates were dried at 80° C. for0.25 h and 180° C. for 1 h in a nitrogen atmosphere and stored in anitrogen atmosphere at room temperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1. In addition, a few regions of slightlydifferent contrast were observed when the analyzer was adjusted slightlyless or greater than 90 degrees (slightly uncrossed from the polarizer)which is due to the flow of the liquid crystal during filling. Thesewere will call flow defects. However, as in Example 1, the background ofthe cell was predominately dark between crossed polarizers indicatingthat the exposure induced the net uniform alignment the liquid crystalscandwiched between the optical alignment layers of this example.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was reported except the scan rate was chosen to be 1.5mm/sec. The same results between crossed and slightly uncrossedpolarizers were observed as described above.

EXAMPLE 4

This example illustrates the formation and use of novel compositions ofthis invention consisting essentially of a copolyimide with structuresof formula IIa and VII.

A mixture of dianhydride D1 (161.1 mg, 0.5 mmol), diamines 1 (35.2 mg,0.20 mmol), 2 (22.0 mg, 0.125 mmol), 5 (40.0 mg, 0.125 mmol), 11 (10.6mg, 0.05 mmol) and γ-butyrolactone (1.78 g was stirred 16 h at roomtemperature under a nitrogen atmosphere. Triethylamine (0.21 mL, 1.5mmol) and a solution of acetic anhydride (0.142 mL, 1.5 mmol) in-butyrolactone (0.80 g) were added sequentially, and the mixture washeated to 120° C. for 3.0 h. The solution was cooled to roomtemperature, diluted to a 3 wt % solution by addition of γ-butyrolactone(6.61 g), filtered through a 0.45 μm teflon filter and spin coated ontosoda-lime glass substrates (0.9″×1.2″) at 2500 rpms. The coatedsubstrates were dried at 80° C. for 0.25 h and 180° C. for 1 h in anitrogen atmosphere and stored in a nitrogen atmosphere at roomtemperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1. Some small regions of different alignment(which we call domains) were also observed between the crossedpolarizers. However, as in Example 1, the background of the cell waspredominately dark between crossed polarizers indicating that theexposure induced the net uniform alignment of the liquid crystalscandwiched between the optical alignment layers of this example.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was repeated except the scan rate was chosen to be 1.5mm/sec. The same results between crossed polarizers were observed asdescribed above. In addition, a few flow defects were observed in thiscell. However, as in Example 1, the background of the cell waspredominately dark between crossed polarizers indicating that theexposure induced the net uniform alignment of the liquid crystalscandwiched between the optical alignment layers of this example.

EXAMPLE 5

This example illustrates the formation of novel compositions of thisinvention consisting essentially of a copolyimide of at least onestructural element of formula IIa and Va.

To a mixture of diamine 1 (88.0 mg, 0.50 mmol) and γ-butyrolactone (2.4mL) was added5-(2,5-dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride, D3, (66.0 mg 0.25 mmol) and the mixture stirred at roomtemperature for 3.5 h under a nitrogen atmosphere. Dianhydride D1 (80.6mg, 0.25 mmol, 98wt %) was added and the mixture heated to 50° C. for 11h and 80° C. for 2.5 h. The mixture was cooled to 40° C., triethylamine(0.35 mL, 2.5 mmol) and a solution of acetic anhydride (0.142 ml., 1.5mmol) in γ-butyrolactone (0.9 mL) were added sequentially, and themixture was heated to 120° C. for 2.5 h. The solution was cooled to roomtemperature, diluted to a 3 wt % solution by addition of γ-butyrolactone(3.1 mL), filtered through a 0.45 μm teflon filter and spincoated ontosoda-lime glass substrates (0.9″×1.2″) at 2500 rpms. The coatedsubstrates were dried at 80° C. for 0.25 h and 180° C. for 1 h in anitrogen atmosphere and stored in a nitrogen atmosphere at roomtemperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was repeated except the scan rate was chosen to be 1.5mm/sec. The same results were observed as described above except thatflow defects were also observed.

EXAMPLE 6

This example illustrates the formation of novel compositions of theinvention.

To a mixture of diamine 11 (106 mg, 0.50 mmol) and γ-butyrolactone (1.16g) was added 1,2,3,4-butanetetracarboxylic dianhydride, D7, (99.0 mg,0.50 mmol) and the mixture stirred at room temperature 16 h under anitrogen atmosphere. Triethylamine (0.209 mL, 1.5 mmol) and a solutionof acetic anhydride (0.142 mL, 1.5 mmol) in γ-butyrolactone (1.0 g mL)were added sequentially, and the mixture was heated to 120° C. for 3 h.The solution was cooled to room temperature, diluted to a 3 wt %solution by addition of γ-butyrolactone (4.16 g), filtered through a0.45 μm teflon filter and spin coated onto soda-lime glass substrates(0.9″×1.2″) at 2500 rpms. The coated substrates were dried at 80° C. for0.25 h and 180° C. for 1 h in a nitrogen atmosphere and stored in anitrogen atmosphere at room temperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. In addition, ZL14792 (EM Industries, Inc.,Hawthorne, N.Y.) liquid crystal was used instead of MLC6043. The sameresults between crossed polarizers were observed as in Example 1 exceptthat some flow defects were also observed when the polarizers wereslightly uncrossed.

EXAMPLE 7

This example illustrates the formation and use of novel compositions ofthis invention consisting essentially of a copolyimide with at least onestructural element of formula VII and VIII.

A mixture of anhydride D3 (99.0 mg, 0.375 mmol),4,4′-diaminobenzophenone (106.1 mg, 0.50 mmol, Aldrich) andγ-butyrolactone (1.39 g) was stirred for 10 min under a nitrogenatmosphere. Dianhydride D1 (40.3 mg, 0.125 mmol) was added and themixture stirred at room temperature. Triethylamine (0.35 mL, 2.5 mmol)and a solution of acetic anhydride (0.142 mL, 1.5 mmol) inγ-butyrolactone (1.31 g) were added sequentially, and the mixture washeated to 120° C. for 3.0 h. The solution was cooled to room temperaturediluted to a 3 wt % solution by addition of γ-butyrolactone (4.92 g),filtered through a 0.45 μm teflon filter and spin coated onto soda-limeglass substrates (0.9″×1.2″) at 2500 rpms. The coated substrates weredried at 80° C. for 0.25 h and 180° C. for 1 h in a nitrogen atmosphereand stored in a nitrogen atmosphere at room temperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1 except that some flow defects were alsoobserved when the polarizers were slightly uncrossed.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was repeated except the scan rate was chosen to be 1.5mm/sec. The same results were observed as described above except that noflow defects were observed.

EXAMPLE 8

This example illustrates the formation and use of novel compositions ofthis invention consisting essentially of a copolyimide with at least onestructural element of formula VIII and IX.

To a solution of 4,4′-diaminobenzophenone, 11, (106.1 mg, 0.50 mmol) andγ-butyrolactone (1.60 g) was added5-(2,5-dioxotetrahydro)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride, D3, (66.0 mg, 0.25 mmol), followed immediately by addition of1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,D2, (111.1 mg, 0.25 mmol). The mixture was stirred at room temperaturefor 16 h under a nitrogen atmosphere. A solution of acetic anhydride(0.142 mL, 1.5 mmol) in γ-butyrolactone (1.0 g mL) and triethylamine(0.209 mL, 1.5 mmol) and were added sequentially, and the mixture washeated to 120° C. for 3 h. The solution was cooled to room temperature,diluted to a 3 wt % solution by addition of γ-butyrolactone (6.24 g),filtered through a 0.45 μm teflon filter and spin coated onto soda-limeglass substrates (0.9″×1.2″) at 2500 rpms. The coated substrates weredried at 80° C. for 0.25 h and 180° C. for 1 h in a nitrogen atmosphereand stored in a nitrogen atmosphere at room temperature until used.

Example 1 was repeated except the glass substrates were coated with thepolyimide of this example. The same results between crossed polarizerswere observed as in Example 1 except that some flow defects were alsoobserved when the polarizers were slightly uncrossed.

In another trial two optical alignment layers of this example wereexposed prior to assembly of the cell and at a higher scan rate. Theabove treatment was repeated except the scan rate was chosen to be 1.5mm/sec. The same results were observed as described above.

EXAMPLE 9

This example illustrates that the mechanical buffing process can be usedfor aligning liquid crystals using a homopolyimide of Example 1.

Coatings on soda-lime glass substrates were prepared using the samematerials and procedures as described in Example 1.

A conventional method of mechanical buffing was used. Two coatedsubstrates were mounted onto custom vacuum chuck made from a block ofaluminum. The coated surfaces were face-up and a vacuum was pulled tohold the substrates in place. The vacuum chuck was inverted such thatthe coated surfaces were face down and subsequently placed on top of afibrous Buehler polishing cloth. The coated surfaces were translatedapproximately 10 inches across the polishing cloth at a constant rateusing only the weight of the vacuum chuck to hold them down onto thecloth. The buffing step was repeated one more time. The vacuum wasreleased and the substrates blown off with 30-40 psi nitrogen gas toremove any particulate contamination.

A liquid crystal cell was constructed from the two rubbed substrates andfilled with MLC6043 nematic liquid crystal as described in Example 1.The results were the same as Example 1 when the cell was viewed betweencrossed polars indicating a net alignment was induced by the mechanicalbuffing process.

EXAMPLE 10

This example illustrates that the mechanical buffing process can be usedfor aligning liquid crystals using a copolyimide of Example 4.

Coatings on soda-lime glass substrates were prepared using thecopolyimide of Example 4. The coated substrates were processed asdescribed in Example 9. The resulting liquid crystal cell showed thesame results as Example 9 except there were no disclination linesvisible in the cell.

EXAMPLE 11

This example illustrates the use of the polyimide of Example 1 in anoptical storage medium.

A cell was prepared as in Example 1 using a 1.5 mm/s scan speed exceptthat ZL14792 was used instead of MLC6043 to fill the cell. The completedcell was subsequently exposed through a metal mask with a 1 mm diameterhole in the center using the optical set-up described in Example 1. Atotal of four exposures at a power of 1 Watt and a scan speed of 0.01mm/s were performed with the 1 mm hole placed in front of differentareas of the cell to prevent overlap of the exposed regions. In eachexposure the orientation of the light polarization in the plane of thecell substrates relative to the background alignment was changed byrotating the cell approximately 0, 22.5, 45, and 67.5 degrees. Thisresulted in the light polarization being approximately at 0, −22.5, −45,and −67.5 degrees, respectively, to the background direction. Since theoptically induced alignment is perpendicular to the light polarization,the induced liquid crystal alignment was expected to be at approximately90, 67.5, 45, and 22.5 degrees, respectively. In addition, since the UVlight was defocussed as well as absorbed and reflected by the glasssubstrate and the liquid crystal, the power was substantially reduced atthe back alignment layer (farthest from the incident light beam)relative to the power at the front alignment layer (closest to theincident light beam). This resulted in the liquid crystal at the frontalignment layer changing orientation while little change occured at theback alignment layer. As a consequence, we observed, by placing the cellbetween polarizers and rotating the polarizer between the viewer and thecell, a twisted nematic orientation of the liquid crystal in the 1 mmexposed regions which corresponded to twist angles approximately equalto 90, 67.5, 45, and 22.5 degrees, respectively. In addition, thedifferent orientations of the 1 mm exposed regions evidenced themselvesby the increasing light transmission with increasing twist angle whenthe cell was viewed between crossed polarizers.

EXAMPLE 12

This example demonstrates that the polyimide of Example 1 can opticallyinduce the alignment of liquid crystals when exposed to polarizedultraviolet lamp light.

Two substrates 14 coated with the polyimide of Example 1 were exposed byan ultraviolet lamp as depicted in FIG. 8. The ultraviolet lamp 15 (UVProcess Supply, Chicago, Ill., Model Porta-Cure 1500F) was 16 cm fromthe substrates 14 with the coated side facing the lamp. A 3×4 inchdielectric polarizer 16 (CVI Laser Corporation, Albuquerque, N. Mex.)was placed in front of the light beam. The polarizer 16 gaveapproximately 20:1 of p-polarized light 11 to s-polarized light intransmission for wavelengths between 300-400 nm. The light wassubsequently passed through a 1 mm thick soda lime glass plate 17(Donnelly Mirrors, Inc., Holland, Mich.). The glass plate 17 has acut-off of approximately 300 nm (transmission is less than 10% for anywavelength less than 300 nm). To prevent illumination of the coatedsubstrates 14 from unpolarized stray light, aluminum foil (not shown infigure) was placed to block all light that did not pass through thepolarizer 16. The output of the lamp 15 was set at 300 Watts/inch andallowed to warm-up for 10 minutes prior to placing the coated substrates14 in front of the light beam.

The power density of the light beam at the substrates 14 was measured tobe 14 milliwatts/cm² using a Control Cure compact radiometer from UVProcess Supply, Chicago, Ill. The substrates were exposed for 10 minutesand a cell was assembled and filled as in Example 1 except that theliquid crystal used was ZL14792.

The results were the same as in Example 1 except for the presence offlow defects when the polarizers were slightly uncrossed. However, as inExample 1, the background of the cell was predominately dark betweencrossed polarizers indicating that the exposure induced the net uniformalignment of the liquid crystal sandwiched between the optical alignmentlayers of this example.

If the liquid crystal displays prepared according to each of the aboveExamples, after optically inducing alignment, are visually examinedunder 100X magnification between crossed polarizers, these liquidcrystal displays will exhibit substantially no irregularities inalignment resulting from scratches, while liquid crystal displayelements treated by mechanical buffing will exhibit substantialirregularities.

TABLE 1 No. Structure Diamines used in Polyimide Alignment Layers 1

2

3

4

5

6

Diamines used in Polyimide Optical Alignment Layers 7

8

9

10 

11 

TABLE 2 Alicyclic tetracarboxylic dianhydrides No. Structure D3

D4

D5

D6

D7

D8

D9

D10 

D11 

We claim:
 1. An optical element comprising at least one substrate, anoptical alignment layer on a surface of the substrate and a liquidcrystal layer on a surface of the optical alignment layer, wherein theoptical alignment layer comprises anisotropically absorbing moleculesconsisting essentially of at least one diaryl ketone and is exposed topolarized light sufficient to align the liquid crystal layer.
 2. In anoptical element of claim 1 in the configuration of a liquid crystaldisplay element comprising two substantially parallel substrates havingfacing surfaces, an alignment layer disposed on each facing surfacewherein at least one of the alignment layers is an optical alignmentlayer, wherein each optical alignment layer comprises anisotropicallyabsorbing molecules; a liquid crystal layer at least partly aligned anddisposed between, and adjacent to, the alignment layers, and a spacerseparating the alignment layers; the improvement wherein theanisotropically absorbing molecules consist essentially of at least onediaryl ketone and wherein the liquid display element is substantiallyfree from irregularities in the alignment pattern when viewed at 100time magnification between crossed polarizers.
 3. An optical element ofclaim 1 in the configuration of an optical storage medium comprising aplurality of substrates, each coated with an optical alignment layer onat least one surface, the coated substrates being separated by a liquidcrystal layer.
 4. An optical element of claim 1 wherein the diarylketone consists essentially of at least one copolyimide of at least onediaryl ketone tetracarboxylic dianhydride and at least three diamines,consisting essentially of at least three structural elements of theformula IIa.

wherein Y is a divalent radical selected form the formulas IIIa and IVa

wherein Z and Z₁ are selected, independently, from the group consistingof —S—, —SO₂—, —O—, — CH₂CH₂—, —CH₂—, —NR—, —C(CF₃)₂—, —C(O)— and acovalent bond, wherein R is a C₁-C₄ hydrocarbon chain; X₂ isindependently selected from —R₃, —OR₃, —SR₃, —N(R₄)R₃; wherein R₃ isselected from C₁-C₃ perfluorinated alkyl chain and partially fluorinatedalkyl chain and R₄ is independently selected from R₃ and H; X₃ isindependently selected from X₂ and H; X is independently selected fromthe group consisting of H, Cl, F, and Br; and m is 1 or
 0. 5. An opticalelement of claim 1 wherein the diaryl ketone consists essentially of atleast one copolyimide of at least one diaryl ketone tetracarboxylicdianhydride and at least two diamines, consisting essentially of atleast one structural element of the formula IIa

wherein Y is a divalent radical selected from the formulas IIIa and IVa

and at least one structural element of formula VII

wherein Z and Z₁ are selected, independently, from the group consistingof —S—, —SO₂—, —O—, — CH₂CH₂—, —CH₂—, —NR—, —C(CF₃)₂—, —C(O)— and acovalent bond, wherein R is a C₁-C₄ hydrocarbon chain; X₂ isindependently selected from —R₃, —OR₃, —SR₃, —N(R₄)R₃; wherein R₃ isselected from C₁-C₃ perfluorinated alkyl chain and partially fluorinatedalkyl chain and R₄ is independently selected from R₃ and H; X₃ isindependently selected from X₂ and H; X is independently selected fromthe group consisting of H, Cl, F, and Br; and m is 1 or 0; and thecopolyimide consists of about from 99 to 1 mol % of at least onestructural element of formula IIa and about from 1 to 99% of at leastone structural element of formula VII.
 6. An optical element of claim 1wherein the diaryl ketone consists essentially of at least onecopolyimide of at least one diaryl ketone tetracarboxylic dianhydride,at least one alicyclic tetracarboxylic dianhydride and at least onediamine, consisting essentially of at least two structural elements ofthe formula IIa and Va

wherein Y is a divalent radical selected from the formula IIIa and IVa

wherein Z and Z₁ are independently selected from the group consisting of—S—, —SO₂—, —O—, — CH₂CH₂—, —CH₂—, —NR—, —C(CF₃)₂—, —C(O)— and acovalent bond, wherein R is a C₁-C₄ hydrocarbon chain; X₂ isindependently selected from —R₃, —OR₃, —SR₃, —N(R₄)R₃; wherein R₃ isselected from C₁-C₃ perfluorinated alkyl chain and partially fluorinatedalkyl chain and R₄ is independently selected from R₃ and H; X₃ isindependently selected from X₂ and H; X is independently selected fromthe group consisting of H, Cl, F, and Br; and m is 1 or 0; P is atetravalent radical derived from said alicyclic tetracarboxylicdianhydride; and the copolyimide comprises 95 to 20 mol % of at leastone structural element of formula IIa and 5 to 80 mol % of at least onestructural element of formula Va.
 7. An optical element of claim 1wherein the diaryl ketone consists essentially of at least one polyimidepolymer consisting essentially of at least one structural element of theformula VIII.

wherein P is a tetravalent optical derived from said alicyclictetracarboxylic dianhydride; and X is independently selected from thegroup consisting of H, Cl, F, and Br.